A key component of an electronic electricity meter is a current transformer that is used as a current sensor. Typical current transformers for this purpose have a core of a magnetic material, a primary conductor axially led through a central hole of the core and constituting a current loop, and a secondary winding galvanically insulated from the primary conductor, the ratio between the primary and secondary being on the order of several thousands. The secondary voltage developed at the terminals of the secondary windings is representative of the primary current, and thus of the electric power consumed. A measurement of an electric energy consumption is obtained by repeatedly sampling this voltage and the mains voltage, multiplying the voltage pairs with each other and with the sampling period and accumulating the multiplication products.
Due to magnetic saturation effects of conventional core materials, current transformers exhibit significant amplitude errors when the current through the primary conductor has a DC component Recently, current transformers have been proposed that tolerate a DC component in the primary current by not producing significant amplitude errors. These current transformers are referred to as DC tolerant current transformers.
DC tolerant current transformers, however, suffer from the problem that they have an inherent phase angle error. This phase angle error causes large measurement errors for phase angles of the load different from zero. For example, when the phase angle error at the nominal mains frequency of 50 Hz is +4.0° and the phase angle of the load is 60.0°, the measurement error amounts to +11.8%. A measurement error of this magnitude is not acceptable. It has been proposed to correct the measurement error with a simple RC combination in the path of the analog voltage signal upstream from the A/D converter. This approach is successful for a constant mains frequency, but ignores the fact that the actual mains frequency may well deviate from the nominal mains frequency.